In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cell stages, each stage comprising a single fuel cell unit, are mechanically stacked up in a “stack” and are electrically connected in series electric flow from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as interconnects and separator plates.
A solid oxide fuel cell (SOFC) comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is mounted, for handling, protection, and assembly into a stack, within a metal frame referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”.
To facilitate formation of a prior art stack of fuel stages wherein the voltage formed is a function of the number of fuel cells in the stack, connected in series, a known intermediate process for forming an individual fuel cell stage joins together a cell-picture frame assembly with an anode interconnect and a metal separator plate to form an intermediate structure known in the art as a fuel cell cassette (“cassette”). The thin sheet metal separator plate is stamped and formed to provide, when joined to the mating cell frame and anode spacers, a flow space for the anode gas. Typically, the separator plate is formed of ferritic stainless steel for low cost.
As part of the assembly process for the cell-picture frame assembly, each cassette is sealed to the perimeter of the metal separator plate of the adjacent cassette to form a cathode air flow space and to seal the feed and exhaust passages for air and hydrogen against cross-leaking or leaking to the outside of the stack. The picture frames may also include openings therein, which provide internal manifolding after the stack has been assembled for the inter-cell flow of air and fuel to and from the intra-cell anode and cathode flow spaces.
The power output P of a fuel cell stack is the product of the voltage V and current I,P=IV  (Eq. 1)
The voltage is a function of the number of fuel cells connected in series in the stack, while the current is a function of the active area of the individual fuel cells. Thus, in designing a fuel cell system, to increase the power output requires an increase in either the number of fuel cells, or the individual fuel cell area, or both.
There are tradeoffs in the number of cells and the surface area of the cells to achieve a desired power level.
Adding more cells in series to increase stack voltage is relatively straightforward, but the reliability of each cell-to-cell connection becomes more critical since the overall reliability of a stack of N cells is a function of the reliability of each connection raised to the Nth power. Also, the resistive losses at the cell-to-cell junctures increase with each connection, and the proportion of system volume required for manifolding of the inlet and return gases increases. Also, in enclosed design stacks where the periphery of the stack is sealed, individual cells in the stack cannot be removed or replaced if they fail, which can result in the failure of an entire stack due to the failure of a single cell. This problem is exacerbated when the stack contains a large number of cells due to the above-described overall drop in stack reliability stacks with large numbers of cells.
On the other hand, increasing the cell active area to increase the stack amperage by increasing the areal extent of each cell presents many challenges. The cell is a planar ceramic structure, so as the size increases the thickness must also increase to preserve the same level of mechanical strength (that is, resistance to breakage) which significantly increases the cost and size (volume) of the cell per unit area of electric generating capacity. In addition, the manufacturing defect rate is determined by the number of defects per cell, not per unit area, so as the area of a cell increases the number of defects per cell will increase, which adversely affects the overall manufacturing rejection rate in both cell manufacturing and stack manufacturing. Also, as the surface area increases at a constant length-to-width ratio (currently preferred aspect ratio of a prior art fuel cell is about 3:2), the thermal differences across the cell will increase, or the pressure drop will increase, or the gas channel height (and thus overall stack height) will increase, or some intermediate combination of all of these effects must occur. Alternatively, the width or length may be increased while maintaining the same length or width, but this departure from a prior art near-square cell shape makes firing of the ceramic cell very difficult while maintaining acceptable flatness and uniform shrinkage.
One approach to increasing power output without unduly increasing either the number cells in a stack or the surface area of the cells in the stack has been to link multiple stacks of manageable size together in a multi-stack fuel cell system. Many approaches to such system designs involve the incorporation of so-called open-cell design fuel cell stacks into a relatively large enclosure that directs fuel and air into stacks housed within the enclosure and receives tail gas and spent air from those stacks for optional further processing and ultimate discharge to the outside. Such systems are disclosed, for example, in U.S. Pat. Nos. 5,480,738 and 5,298,341, and U.S. Pat. Appl. Publ. No. 2009/0053569 A1. Such systems, however, suffer from a number of disadvantages. For example, because open-cell design stacks are open to whatever gas they are exposed to, the enclosure must often provide multiple sealed connections to the stacks so that air can be selectively directed to and spent air selectively received from cathode openings on the stack, and so that fuel can be selectively directed to and tail gas selectively received from anode openings on the stacks. Such seals must be maintained across multiple fuel cells in the stacks, which makes it difficult to account for thermal expansion while maintaining a gas-tight seal. Additionally, since the cells in a stack are connected in electrical series, they are at different voltages, so the seal must be electrically insulating, which can make it even more difficult to establish and maintain a gas-tight seal through multiple thermal cycles. Some design approaches reduce the requirement for sealing surfaces across multiple cells by using alternative flow patterns within the stack. For example, the above-cited U.S. Pat. No. 5,480,738 provides for vertical fuel flow upwards through the stack while having horizontal air flow across the cell cathode surfaces. This design, however, produces a cross-flow arrangement of fuel and air flows across the fuel cells, which causes high thermal gradients and therefore high stress on the cells. In addition, a vertical cell orientation is less resistant to external forces such as gravity and vibration coming from the surrounding environment, which may be better absorbed by the cell and supporting structures in a horizontal arrangement. Also, external manifolding arrangements are difficult to seal due to irregular surfaces on the sides of the stacks inherent in this layered construction.
The above-described disadvantages with open-cell stack designs can be avoided by an enclosed cell stack design with internal manifold channels for fuel and air flow as described, for example, in U.S. Pat. No. 7,306,872. Heretofore, however, there has not been a way to effectively connect multiple enclosed cell design stacks without the use of complex external manifolds.
What is needed in the art is a means to increase the power output of a fuel cell system while mitigating the occurrence of the above-identified problems.